Strain Fluctuations from DIC Technique Applied on Paper

We study the evolution of deformation on a quasi 2D paper sample during fatigue and creep experiments. Paper is a fibrous material, sensitive to various parameters (humidity, temperature, history effect). As for many materials, the lifetime varies a lot. Basquin's law claims that the number of cycles at break in fracture tests has a power-law dependence on the external load amplitude; Monkman-Grant relationship predicts that lifetime is proportional to the minimum strain rate during creep. The main question is therefore: is it possible to predict the time-dependent rupture? We perform creep and cyclic tests on paper samples. The main tools of investigation are acoustic emission (AE) -based analysis and digital image correlation (DIC). Via the AE, one can follow the damage evolution in samples, like microcrack formation that releases elastic energy. The DIC instead works well as a tool to measure at great accuracy both global and local deformation fields. From these, local strain rates can be derived, which then can be used to understand the microscopic dynamics that lie behind a certain rheology or sample response. The issue is to predict the break by looking at the sample-to-sample variation and by comparing experiments.